The Strange Ways of the Cutlip Minnow

During regular stream sampling, researchers noticed distinct humps of reddish-orange pebbles in Ontario’s Rotary Creek. Although neither are fisheries scientists, Dr. Andy Bramburger, Research Associate at University of Minnesota Duluth’s Natural Resources Research Institute, and Dr. Brian Hickey of the St. Lawrence River Institute of Environmental Sciences took it upon themselves to investigate the phenomenon. Bramburger is an algal ecologist and Hickey is a bat conservation biologist, but this study was too interesting for either to turn down.

The humps turned out to be nests made by male cutlip minnows, which get their name from their distinctive, specialized mouths. Their lower jaw has a central bony plate flanked by two fleshy lobes, giving them the appearance of a cut lip and allowing them to use their mouths in unorthodox ways. “In the stream we studied, we saw many yellow perch and rock bass who were missing one or both eyes,” stated Bramburger, who shared that the males will pluck the eyes out of fish that try to eat their eggs. These unusual mouths are not only handy weapons, they also allow males to pick up and move pebbles to form nests.

A study co-authored by Bramburger and Hickey, published earlier this year, focused on how male cutlip minnows choose which pebbles to use in their nest by color. In fact, every single nest fell into an incredibly narrow range of average wavelengths, meaning that they all were almost the exact same shade of red-orange. “Obviously, the pebbles available to male cutlip minnows on the streambed are not uniform in color. Each male fish essentially has to make a mosaic from different color pebbles so that the average color of the mosaic is within a narrow range that females will like,” said Bramburger, “The fact that all the nesting males in this population achieved nearly identical dominant wavelengths in their nests was remarkable.” In addition, the pebbles used by cutlip minnows in their nests are darker, and more heavily pigmented than streambed backgrounds, suggesting that the males aim to build highly conspicuous nests.

Many minnow species are sexually dimorphic, meaning that the males and females look different, even if only during the mating season. In contrast, cutlip minnow males and females look the same. Instead of choosing a mate based on his flashy scales, female cutlip minnows seem to be choosing males based on their ability to build a perfectly colored nest. “In this species, nest coloration provides female fish with important information about the genetic health of the male who built the nest,” explained Bramburger. He went on to say that male minnows usually display red coloration during spawning season, so it is probable that female minnows have evolved to respond to these colors when it comes time to mate.

Unfortunately, this particular adaptation may put the species at risk. If a stream’s water quality becomes compromised, suspended sediments in the water column could obstruct the fish’s vision. Not only would this affect the males’ abilities to find nesting material, it would affect the females’ abilities to evaluate the nests. “Many fish have highly ritualistic mating behavior, and if a single cue in the ritual, like an appropriately colored nest, is missing, mating may not take place,” said Bramburger.

Cutlip minnows are already a threatened species. They would be one of the first to suffer if their ecosystem became impaired. Not only is this a worrying concept just on principle, it would be especially disappointing to lose this species to extinction because of its uniqueness. As of now, cutlip minnows are the only reported example of fish choosing nest material based on color. They may very well have opened a door into an entirely new area of fisheries research.

In the future, Bramburger hopes he and other scientists can pursue this line of study. He’s interested in learning if other cutlip minnow populations build nests out of similarly-colored material and also if other species of fish choose nesting material based on color.